| 1. |
- Nilsson, J. A.
(author)
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Clutch size determination in the marsh tit (Parus palustris)
- 1991
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In: Ecology. - : Wiley. - 0012-9658. ; 72:5, s. 1757-1762
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Journal article (peer-reviewed)abstract
- By providing supplemental food to Marsh Tits (Parus palustris) from the day the first egg was laid, effects on clutch size could be directly attributed to food without the confounding effects on laying date. Females provisioned with additional food responded by laying, on average, on egg more than control females. Differences in territory quality did not influence this result because, on territories occupied in both years, one more egg was laid in the year of provisioning. Although the rate and amount of extra food provided were equal during the two experimental years, the resulting clutch sizes of provisioned females differed between years by more than one egg in absolute terms. Further, the mean clutch size of provisioned females declined seasonally in parallel to that of unprovisioned females. I conclude that clutch size in Marsh Tits is determined by a tactical decision based on declining values of offspring with the progress of the season. However, within the limits sets by this tactical decision, the availability of food during egg—laying also plays a role.
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| 2. |
- Nilsson, Jan-Åke, et al.
(author)
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Energetic constraints and ultimate decisions during egglaying in the blue tit
- 1993
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In: Ecology. - : Wiley. - 0012-9658. ; 74:1, s. 244-251
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Journal article (peer-reviewed)abstract
- In a population of Blue Tits, Parus caeruleus, we performed two different food provisioning experiments; one starting before clutch initiaition and one starting when the first eggs were laid. We assessed the effect of these feeding experiments on four fitness—related factors, viz., laying date, clutch size, egg mass, and onset of incubation. Given that breeding birds are food and energy constrained, extra food during the breeding season should be invested in the fitness—related factor(s) that is most important for maximization of overall fitness. In the first feeding experiment, when breeding pairs had access to extra food both before and during egg—laying, the female converted this extra energy into earlier laying dates. When food was provided after clutch initiation only, the above preferred option was no longer available. In this situation, females used the extra energy to start incubating earlier in the laying sequence, with no influence on the other factors studied. Thus, both experiments resulted in surplus energy being used to increase the quality of the offspring instead of their number. In both cases the quality was connected with time savings leading to earlier hatching dates.
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| 3. |
- Åberg, Per, 1959
(author)
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A DEMOGRAPHIC-STUDY OF 2 POPULATIONS OF THE SEAWEED ASCOPHYLLUM-NODOSUM
- 1992
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In: Ecology. - : Wiley. - 0012-9658. ; 73:4, s. 1473-1487
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Journal article (peer-reviewed)abstract
- Very little is known about the demography in size-structured seaweed populations, and this is especially true for populations in variable environments. Thus, the demography of the brown alga Ascophyllum nodosum was analyzed with a matrix population model. This was built on a 3-yr study in two populations on the Swedish west coast, where > 1100 individuals were marked and followed twice each year. The environmental variability is due to the presence of ice during some of the winters. The frequency of ice years in the study area is known, and is higher at one site compared to the other. During the study there were 2 yr with ice and one without at both sites, and these temporal changes in the environment resulted in large variations in the vital rates of A. nodosum. The individuals were divided into five size classes and the population dynamics at years without ice were characterized by low mortality rates and high transition probabilities for growth to larger sizes, while years with ice had high mortality rates and high transition probabilities for breakage to smaller sizes. Of the 25 possible transitions in the life cycle graph all except 2 had nonzero entries, which means that the adult life of A. nodosum individuals can be described as plastic growth between all five size classes. A crude estimate of the recruitment showed that both populations will increase in numbers in ice-free years, but will decrease in years with ice. The size-dependent fertility rate is probably subject to errors, and thus the survival matrices were scaled with a fertility function built on total reproductive biomass per individual, allowing analyses of the demography at different levels of the asymptotic growth rate lambda(1). One of the main differences between the populations was that the stable size distribution for survival matrices in ice-free years was dominated by size class 4 at one population but class 5 at the other. Thus, to achieve the same value of lambda(1) higher fertility rates were needed at the population dominated by class 4. Elasticity analysis of the transition matrices showed the same trend, although the main result from that analysis was that growth to larger sizes or remaining in the same class contributed more to lambda(1) than reproduction. This was valid for all levels of lambda(1) investigated.
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| 4. |
- Åberg, Per, 1959
(author)
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SIZE-BASED DEMOGRAPHY OF THE SEAWEED ASCOPHYLLUM-NODOSUM IN STOCHASTIC ENVIRONMENTS
- 1992
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In: Ecology. - : Wiley. - 0012-9658. ; 73:4, s. 1488-1501
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Journal article (peer-reviewed)abstract
- Large variations in vital rates are found among individuals in populations of Ascophyllum nodosum along the Swedish west coast due to temporal variation in the environment (years with or without ice). The population growth in these stochastic environments was analyzed for two populations using model simulations. A matrix model with a set of three population projection matrices for each population described the demography during three different types of years (environments), i.e., years with (1) no ice, (2) moderate damage due to ice, (3) large damage due to ice. The frequency of ice years is known for the area studied and was higher at one site (Goteborg) compared to the other (Tjarno). These frequencies were used as the stochastic process generating the sequence of matrices. The mean mortality rates decreased with increasing size in both populations. Simulating the growth for survival matrices (no reproduction included) resulted in an average population growth rate lambda(s) of 0.84 at Tjarno and 0.76 at Goteborg. The average growth rate lambda(s) was set to 1 by scaling the set of matrices for each population with a fertility function. To achieve lambda(s) = 1, almost-equal-to 3.2 times more fertility in each size class was required at Goteborg. Because of the large environmental variability, lambda(s) varied from 0.3 to 1.3 during a long stochastic sample path. The maximum lifetime of A. nodosum individuals was estimated to be almost-equal-to 50-60 yr, which probably is shorter than for areas with no ice. The population growth with the sets of scaled matrices (reproduction included) was simulated by following a large number of subpopulations for 300 yr. With extinction defined as the first passage time through the point N = 1, almost-equal-to 25% of the subpopulations "died" during the simulation, and the mean extinction time for those who died was 163 yr. An elasticity analysis of lambda(s) to changes in the matrix elements showed that at each time step is was most important for A. nodosum individuals to survive and grow to the next larger size class or to survive and remain in the same size class.
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